Various efforts have hitherto been made to increase the functionality of polymeric compounds. For example, in one approach currently used to increase the refractive index of polymeric compounds, aromatic rings, halogen atoms or sulfur atoms are introduced onto the compound. Of such compounds, episulfide polymeric compounds and thiourethane polymeric compounds, both of which have sulfur atoms introduced thereon, are in practical use today as high-refractive index lenses for eyeglasses.
However, given that material design to a refractive index above 1.7 is difficult with a polymer alone, the most effective method for achieving an even higher refractive index is known to involve the use of inorganic fine particles.
This method is a technique for achieving a higher refractive index by mixing together a polymer and inorganic fine particles. The mixing method generally entails mixing a polymer solution with a dispersion of inorganic fine particles, in which case the polymer serves as a binder which stabilizes and keeps the dispersion of inorganic fine particles from breaking down.
It has been reported that polysiloxanes and polyimides can be used as the binder polymer.
For example, a method for increasing the refractive index by using a hybrid material composed of a polysiloxane mixed with a material containing a dispersed inorganic oxide such as zirconia or titania has been disclosed (Patent Document 1).
A method for increasing the refractive index by using a hybrid material composed of a polyimide mixed with an inorganic oxide or sulfide material containing dispersed titania, zinc sulfide or the like has also been disclosed (Patent Document 2).
These hybrid materials have been modified in various ways to increase the refractive index. However, when the refractive index of the binder polymer and the refractive index of the inorganic fine particles are compared, the refractive index of the inorganic fine particles is generally higher.
Hence, an effective way for increasing the refractive index of the hybrid material even further would be to increase the refractive index of the lower refractive index component; i.e., the binder polymer.
This has led to the disclosure of, for example, a method for introducing condensed ring structures having a high refractive index onto portions of the polysiloxane (Patent Document 3), and a method for introducing sites that increase the electron density onto portions of the polyimide (Patent Document 4).
However, even in such binder polymers that have been modified to increase the refractive index, the refractive index is currently about 1.6 to 1.7, which is still lower than that of inorganic fine particles, which have refractive indices of about 1.8 to 2.1.
Hence, further increasing the refractive index of the binder polymer to more than 1.7 is an important element for achieving a higher refractive index in hybrid materials.
In recent years, there has arisen a need for high-performance polymeric materials in the development of electronic devices such as liquid-crystal displays, organic electroluminescent (EL) displays, optical semiconductor (LED) devices, solid-state image sensors, organic thin-film solar cells, dye-sensitized solar cells and organic thin-film transistors (TFT).
The specific properties desired in such polymeric materials include (1) heat resistance, (2) transparency, (3) high refractive index, (4) high solubility, and (5) low volume shrinkage.
However, because the high refractive index lens materials for eyeglasses mentioned above generally have a poor heat resistance, requiring that production be carried out in a temperature range no higher than 200° C., materials of this type are unsuitable for processes such as baking in open air at 300° C.
Moreover, because polymeric compounds in which aromatic rings or triazine rings have been introduced generally have an inadequate solubility in solvents, they are insoluble in resist solvents which are safe solvents. On the other hand, materials which exhibit a high solubility generally have a low transparency.
Although triazine ring-containing hyperbranched polymers synthesized as polymers for use as flame retardants have been reported in the literature (Non-Patent Document 1), there are no reports of such hyperbranched polymers being hybridized with inorganic fine particles to form compositions.
As used herein, “hyperbranched polymer” refers to a highly branched polymer with an irregular branched structure that is obtained by, for example, polymerizing ABx-type polyfunctional monomers (where A and B represent functional groups that react with each other, and “x” on B is a number equal to 2 or more). Highly branched polymers include also the polymers having a regular branched structure that are referred to as “dendrimers.” However, hyperbranched polymers are characterized by being easier to synthesize than dendrimers, and by the ease with which high-molecular-weight bodies can be synthesized.